A heat treatment for killing and/or inactivation of microorganisms can be accomplished by means of moist as well as dry heat. In the food industry the heat treatment in moist heat is preferred since the biological killing and inactivating mechanisms are much more effective at a high water content than at a low water content, i.e. dry heat. Besides, the heat transfer in the equipment used for the heat treatment is much more effective with moist heat.
In a package material having a base layer of paper or board this layer must be made relatively thick in order to give the container a necessary dimensional rigidity. However, containers made of these laminates based on paper or board have the drawback of rapidly losing their mechanical strength properties when they are subjected to liquid or moisture, which results in that the container becomes flabby and cumbersome. Thus, such known containers made of a packaging laminate with a liquid absorbing fibre layer cannot be subjected to a heat treatment with moist heat without the desired mechanical rigidity of the packaging laminate and thus the dimensional stability of the container being impaired or lost.
In order to avoid these problems packaging laminates have been developed which are adapted to withstand drastic conditions in the form of moisture and/or heat. Such a known alternative package material has for example a strengthening base layer of plastic and a filler intermixed with the plastic. This material has proved to be less moisture sensitive than the previously used materials based on paper and board. Containers manufactured from these laminates are very suitable for preservation by means of refrigeration as well as heat. When a preservation by means of refrigeration is used these containers with a solid and/or liquid filling material can be chilled by means of water cooling.
However, paper and board are cheap package materials, and containers have thus been developed which comprise a laminate with a base layer of one of these materials. Containers manufactured from such a material of laminate type in the form of a sheet or a web can be heat treated in a humid atmosphere at high temperatures.
These laminates can by means of folding be used for the manufacture of dimensional stable impermeable packaging containers which are very suitable to be used at most extreme conditions in a humid environment including heat treatment with moist heat at an overpressure. Such harsh environments comprise autoclaving at temperatures and periods which are accepted for foods. This means that the laminate is also highly suitable to be used for the manufacture of a container which is intended to be filled with a food under aseptic conditions. In this connection the product is sterilized and filled under almost sterile conditions in a likewise sterilized container which after filling is sealed in such a way that the filled product during storage is not reinfected by microorganisms before it is consumed.
Such a heat treatment of the containers is achieved with a heat transfer medium containing water. Generally, a heat treatment with moist heat is utilized at overpressures in autoclaves which are filled with water or steam. In this connection an autoclaving cycle can be divided into a rise time, a holding time, and a cooling time. The rise time is the time from the beginning of the heating and until the desired temperature has been reached. At an overpressure of 0.5 bar the steam has a temperature of about 110.degree. C., and at an overpressure of 1.1 bar the temperature becomes about 121.degree. C. After the holding time, which is the time required to achieve a sterilization at the desired constant temperature, the pressure in the autoclave is lowered to a normal atmospheric pressure, and during the cooling time the temperature is lowered by means of cooling the autoclave.
Since autoclaving is a sterilization method which uses an overpressure this method is mostly performed as a batch procedure and not continuously. It is thus of interest to be able to reduce the cycle time when autoclaving.
However, it has become apparent that when the above-mentioned containers are too extensively heat treated they are subjected to environments whereby the good sealing properties of the containers against moisture and liquid partly are lost. Due to the soaking paper or board layers it is in this way a risk that the package material loses its mechanical strength properties and that the container becomes flabby and cumbersome. It is thus important that this type of containers is exposed to such environments to the smallest extent possible.